The field of the invention pertains to solar cells and to methods of manufacturing such solar cells.
Great effort has been and continues to be expended on the wringing of critical increments of efficiency out of solar cells. The reason, of course, is the hope that solar cells can, some day, displace a substantial amount of the more conventional energy sources.
The details of the electrical contacts on the solar cells have been viewed as an area to be pursued.
Traditionally, lines of contact material have been employed in forming the front contact. The idea has been to balance the requirement for space between contact areas for the passage of the solar energy into the semiconductor for conversion, with the requirement that contact areas be sufficiently close to one another to collect charges (representing the electrical energy converted from the solar radiation) reaching the front surface of the cell without unacceptable losses due to travel of the charges along the higher resistance non-contact areas to reach the low resistance contact areas.
With regard to the rear contact, the most traditional form remains a rear contact that essentially covers the whole rear of the solar cell. This avoids any travel of the charges at the rear of the cell along non-contact areas.
Solar cells having this now traditional form are illustrated in the Arco Solar, Inc. brochure Arco Solar Photovoltaic Modules, 1982. The cells in the brochure are circular except that they are cut, for handling purposes, along two parallel chords of the circle. The front contact grid has a perimeter contact portion having the shape of the cell. Further, there is a group of contact lines across the cell between opposite parts of the perimeter portion, these lines running in the same direction as the cut-offs; two lines perpendicular to this group of lines, crossing all of the lines of the group except a relatively small number at each end of the group, and terminating before the perimeter portion of the contact at each of their ends; and a straight line at each end of each cross line extending from such end toward the midpoint of the respective cut-off and terminating at the contact perimeter portion. All of these lines essentially are the same width. The contact also has connection elements in two rows running in the same direction as the just-described cross lines, serving as positions to connect the front contact of the cell to the back contact of the next cell in an array of cells. In such an array, elongated metallic connectors are soldered to these elements for that purpose.
It is noteworthy that the positioning and soldering of the elongated connectors is an area which is somewhat critical to obtaining an efficient array manufacturing process--particularly where this process is automated. A common problem is the misalignment of the elongated connectors and missed contact of the connectors with some of the connection elements. Such missed connections make the missed connection elements ineffective and thereby impinge on the charge collection process and on the efficiency of the solar cell.
As to solar cells of a generally rectangular or square shape, the Solavolt International brochure (1982) for that company's MSP13E10, MSP23E20 and MSP43E40 solar cell modules is illustrative in, e.g., showing and describing a front contact pattern which apparently is based upon the concept of a group of parallel contact lines accompanied by the continuous bonding of elongated connectors across the front of the cell and the lines. For purposes of soldering of the connectors, and perhaps the charge collection, although not shown or described, it is understood that segmented cross-contact lines, crossing the group of parallel lines, are present under the elongated connectors.
What is somewhat in the nature of a variation on the just-described concept, which has been the subject of some attention, is a front contact predicated upon the concept of a parallel group of contact lines accompanied by crossing contact portions in the general shape of a curved "W" having width variations, with elongated connectors soldered to the base areas of the "W". Such attention is understood to be in respect to experimental large-sized cells.
As to variations on the traditional solid rear contacts, efficiency considerations have led to an appreciation for certain advantages that can be derived from rear contacts which only partially cover the backs of the cells. At the same time, there has been an appreciation that the potential sacrifice in collection efficiency typically is greater at the back than at the front.
Concerning such sacrifice, in the typical silicon solar cell, there is a front doped semiconductor layer having one conductivity type and a rear doped semiconductor layer of the opposite conductivity type, such layers forming the P/N junction where they come together. The rear layer, on which the rear contact is disposed, will typically have a significantly lower conductivity (higher resistivity) than the front layer. The result is a greater loss due to charges travelling along the rear layer material to reach partial contacts at the rear of the cell.
As to the potential advantages, with respect to the just noted general type of solar cell, these relate to the typical conversion efficiency versus wavelength for such cells. Specifically, the capability to receive solar radiation and convert it to electrical charges (and thus electrical energy) largely exists below the upper limit of visible radiation--i.e. below a wavelength of about 0.7 microns. There is some, less pronounced, effectiveness in the near infrared range--i.e. from about 0.7 microns to about 1.5 microns. However, beyond this there is considered to be essentially no effective conversion. Specifically, such radiation essentially passes through the semiconductor material and presents only the disadvantage of being absorbed in the rear contact material and of heating the rear contact material and the semiconductor. Such heating decreases the efficiency of conversion in the semiconductor by in the range of one-half percent for each degree Centigrade. By the standards of present day concerns with solar cell efficiency, for example, a 3 to 5-degree Centigrade decrease in temperature is a substantial improvement.
M. M. Koltun, Selective Optical Surfaces for Solar Energy Converters, 1979 (Russian) (1981 English translation) devotes substantial attention to partial back contacts (along with the partial front contacts) to permit the exit of infrared radiation. See Chapter 1.3, "Optimization of the Parameters of Semiconductor Photocells Transparent Beyond the Long-Wave Edge of the Fundamental Absorption Band", pp. 27-37 (particularly p. 29 and pp. 33-37); Chapter 2.2, "Temperature Stabilization and Shielding of Silicon Cells From Radiation By Optical Coatings", pp. 85-111 (particularly pp. 101-107); Chapter 2.3, "Prospects", pp. 111-116 (particularly pp. 113-115). Such material contains somewhat detailed resistance considerations in terms of rectangular or square cells and of contacts having a perimeter portion and lines of a given width with a given spacing therebetween. It also looks toward partial contacts occupying less than about ten percent in surface area.
M. Giuliano and J. Wohlgemuth, "The Gridded Back Contact And Its Effect On Solar Cell Performance", 15th IEEE Photovoltaic Conference Proceedings, April, 1981, pp. 111-114, is similarly directed to enhancing performance from a partial back contact. However, it attributes such enhancement to another phenomenon--reflection at the back of the solar cell (as opposed to absorption by a solid contact) of radiation in the vicinity of the near infrared range, in effect, to give the semiconductor material another chance to convert such radiation to electrical charges and energy. The work described is for a particular type of cell having a back surface field (BSF). The increased efficiency is seen as being due to reflection possibly stemming from surface texturing and being dependent upon the formation process and materials connected with the back surface field. In this regard, it is noteworthy that reflection of the near infrared creates more significant advantages in thinner cells where there is less chance for absorption of the near infrared during its initial passage through the semiconductor material. It is also noteworthy that back surface fields generally are thought useful for semiconductor materials having resistivities of 5 ohm centimeters or greater.
In the Giuliano et al. material, the experimentation is with square cells having front and back partial contacts. The back contact grid apparently is a group of four-legged zigzag lines with two elongated connectors across them leading into a thick rectangular end connector along one end of the rear surface. The front grid apparently is a group of six-legged zigzag lines with three elongated connectors across them and leading into another elongated connector perpendicular to the first connectors near one end of the front surface. There is then a thickened connector block along this other elongated connector near one corner of the surface.
On another related matter, concerning the shaping of solar cells, such shaping can typically result in significant sacrifices in the efficiency of the manufacturing process. Specifically, the sacrifice of material during the shaping and the additional handling of the cells in connection with the shaping can both be costly. With the concern for competitiveness of solar cells with traditional energy forms, this sacrifice in cost is a significant concern.
The present invention addresses in a detailed and comprehensive manner the efficiency considerations connected with the rear contact and with the front contact of a solar cell. It particularly addresses them in the context of a solar cell which is shaped for efficient packing in an array of like solar cells. The achieving of such shape, from an ingot, during the manufacture of the solar cell is also addressed in a convenient, efficient fashion.